Films of dielectric materials have been observed to be capable of acquiring a static electric homo-charge upon being poled (i.e. being exposed to a field of high voltage or to a corona discharge), which static charge is quasi-permanent, the degree of permanency being dependent largely upon the chemical constitution of the film. Such a charge, due largely to trapped positive and negative carriers positioned at or near the two surfaces of the dielectric respectively, produces what is known as an electret. When the opposing surfaces of an electret are not covered by metal electrodes, an electret produces an electrostatic field and is in a sense the electrostatic analogue of the permanent magnet.
In addition to being capable of acquiring a static electric charge (and, as a result acquiring an electrostatic field) certain films also acquire an ordered internal molecular orientation when poled, known generally as polarization so that even when the static charges resulting from the trapped charges are dissipated, the film thereafter possesses electric charge-producing properties known as piezoelectric and pyroelectric effects (i.e. the property of producing electron flow between the electrically connected opposed surfaces, by changing the pressure imposed on the opposed surfaces or by changing the temperature of the film). Unfortunately, the term "electret" has been broadly used in the art to indicate any film that has been poled whether or not there has been molecular orientation. Such a broad application of the term is misleading. The confusion is compounded by the fact that piezo-and pyroelectric films are capable of acquiring static electric charges along with the internal molecular orientation they undergo during poling. The process of the present invention is directed to the production of piezoelectric and pyroelectric film and requires relatively rigid control of poling conditions and of the choice of film employed. The use of corona discharge to produce piezoelectric and pyroelectric film is known in the art. The use of the textured surface on at least one electrode in conjunction with the use of a backup pellicle during corona discharge is the crux of the present invention. The process permits the production of thin piezoelectric and pyroelectric film possessing relatively high piezoelectric and/or pyroelectric properties.